This is yet another sign of the collapse of the American middle class, and it's a bad omen for the American political system. We increasingly lack a shared culture or shared experiences, and that makes democracy a tough act to pull off. The well-off have less and less interaction with the poor outside of the market economy, and less and less empathy for how they live their lives. For too many of us, the "general welfare" these days is just an academic abstraction, not a lived experience.

He does not give a reason, and apparently following the links, neither does the study author. But my guess is that they might well attribute it to 1. effects of racism, 2. growth of the suburbs, 3. laissez faire capitalism.

I don't think racism can be the driver of this change, given that racism and fear of other cultures is demonstrably better in the last 30 years than at most times in history (read bout 19th century New York if you are not sure). The suburbs have been a phenomenon for 100 years or more, and capitalism has been less laissez faire over the last 30 years than at any time in our history.

I actually believe a lot of this income sorting is a direct result of two progressive policies. I have no data, of course, so I will label these as hypotheses, but I would offer two drivers

Strict enforcement of the public school monopoly. People want good schools for their kids. Some are wealthy enough to escape to private schools. But the only way for those who stay in the public school system to get to the best schools is to physically move into their districts. Over time, home prices in the best districts rise, which gives those schools more money to be even better (since most are property tax funded), and makes them even more attractive. But as home prices rise, only the most wealthy can afford them. This is dead easy to model. Even in a starting state where there are only tiny inhomegeneities between the quality of individual schools, one ends up with a neighborhood sorting by income over time. Ex post facto attempts to fix this by changing the public school funding model and sending state money to the poorest schools can't reverse it, because at least half of school quality is driven not by money by by the expectations and skills of the parents and children in it. Thus East St. Louis can have some of the highest per pupil spending in the state but have terrible schools. A school choice system would not likely end sorting by school, but it would eliminate a huge incentive to sort by neighborhood.

Strict zoning. There has always been a desire among certain people to exclude selected groups from their neighborhoods. This desire has not changed, or if anything I would argue it has declined somewhat. What has changed is the increased power that exists to exclude. Zoning laws give the rich and well-connected the political vehicle to exclude the rabble from their neighborhoods in a way that never would have been possible in a free market. I live just next to the town of Paradise Valley, which has very strict zoning that is absolutely clearly aimed at keeping everyone but the well-off out. They will not approve construction of new rental units. The minimum lot sizes are huge, way beyond the reach of many.